U.S. patent number 7,796,314 [Application Number 10/592,035] was granted by the patent office on 2010-09-14 for method and apparatus for two-axis, high-speed beam steering.
This patent grant is currently assigned to Board of Regents of the Nevada System of Higher Education. Invention is credited to Nelson George Publicover, John L. Sutko.
United States Patent |
7,796,314 |
Sutko , et al. |
September 14, 2010 |
**Please see images for:
( Certificate of Correction ) ** |
Method and apparatus for two-axis, high-speed beam steering
Abstract
Dual axis, beam-steering devices are disclosed. An exemplary
device includes a support platform having a top surface. A
reflective surface is coupled to the top surface of the support
platform. First and second galvanometers are coupled via respective
linkages to the support platform such that the first galvanometer
rotates the support platform about a first rotational axis, and the
second galvanometer rotates the support platform about a second
rotational axis that is orthogonal to the first rotational axis.
The support platform can be simultaneously rotated with respect to
both the first rotational axis and the second rotational axis to
steer a beam of electromagnetic energy (e.g. light beam) reflected
by the reflective surface.
Inventors: |
Sutko; John L. (Reno, NV),
Publicover; Nelson George (Reno, NV) |
Assignee: |
Board of Regents of the Nevada
System of Higher Education (Reno, NV)
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Family
ID: |
34976194 |
Appl.
No.: |
10/592,035 |
Filed: |
March 8, 2005 |
PCT
Filed: |
March 08, 2005 |
PCT No.: |
PCT/US2005/007759 |
371(c)(1),(2),(4) Date: |
September 13, 2007 |
PCT
Pub. No.: |
WO2005/086858 |
PCT
Pub. Date: |
September 22, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080123169 A1 |
May 29, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60551655 |
Mar 8, 2004 |
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Current U.S.
Class: |
359/198.1;
359/199.1; 359/212.2; 359/214.1; 359/224.1; 359/199.4 |
Current CPC
Class: |
G02B
21/0048 (20130101) |
Current International
Class: |
G02B
26/08 (20060101) |
Field of
Search: |
;359/198.1-202.1,213.1-215.1,223.1,224.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Peng, S. et al., "Diffusion of Single Cardiac Ryanodine Receptors
in Lipid Bilayers is Decreased by Annexin 12," Biophysical Journal,
vol. 86 (Jan. 2004), pp. 145-151. cited by other .
Peng, S. et al., Imaging Single Cardiac Ryanodine Receptor Ca2+
Fluxes in Lipid Bilayers, Biophysical Journal, vol. 86 (Jan. 2004),
pp. 134-144. cited by other .
International Search Report for related application PCT/US05/07759,
ISA/US, 5 pp., mailed Feb. 22, 2007. cited by other.
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Primary Examiner: Phan; James
Attorney, Agent or Firm: University of Nevada, Reno,
Technology Transfer Office Heck; Ryan A.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This is the National Stage of International Application No.
PCT/US2005/007759, filed Mar. 8, 2005, which claims the benefit of
U.S. Provisional Patent Application No. 60/551,655, filed Mar. 8,
2004, all of which are incorporated herein by reference.
Claims
What is claimed is:
1. A two-axis, beam-steering device, comprising: a support platform
having a top surface; a reflective surface situated on the top
surface of the support platform; a first galvanometer having a
first rotational shaft which defines a first fixed rotational axis;
a second galvanometer having a second rotational shaft which
defines a second fixed rotational axis oriented substantially
orthogonally to the first rotational axis; a first linkage
extending along the first rotational axis and coupling the first
galvanometer to the support platform; and a second linkage
extending along the second rotational axis and coupling the second
galvanometer to the support platform, the first galvanometer being
situated relative to the support platform so as to rotate the
support platform about the first rotational axis, and the second
galvanometer being situated relative to the support platform so as
to rotate the support platform about the second rotational axis,
thereby providing simultaneous rotation of the support platform
about the first and second rotational axes as the reflective
surface reflects a beam of electromagnetic energy incident to the
reflective surface.
2. The beam-steering device of claim 1, wherein: the reflective
surface has a center; and the first and second rotational axes
intersect at the center.
3. The beam-steering device of claim 1, further comprising a
position sensor situated and configured to determine a position of
the reflective surface relative to a fixed reference.
4. The beam-steering device of claim 3, wherein the position sensor
is situated and configured to determine the position of the
reflective surface relative to the fixed reference comprising the
first and second rotational axes.
5. The beam-steering device of claim 3, wherein the position sensor
comprises: a first rotational-position sensor situated and
configured to determine a rotational position of the first linkage
about the first rotational axis; and a second rotational-position
sensor situated and configured to determine a rotational position
of the second linkage about the second rotational axis.
6. The beam-steering device of claim 5, wherein the first and
second rotational position sensors produce respective positional
feedback data from which a position of the reflective surface is
determined.
7. The beam-steering device of claim 3, wherein the position sensor
comprises an optical detector situated and configured to detect
electromagnetic radiation reflected from the reflective surface, to
produce corresponding light-position data, and to correlate the
light-position data with a corresponding position of the reflective
element.
8. The beam-steering device of claim 3, wherein the position sensor
comprises a capacitance detector situated and configured to obtain
data regarding electrical capacitance between the support platform
and the fixed reference.
9. The beam-steering device of claim 8, wherein the capacitance
detector comprises at least one electrostatically charged silicon
comb situated between the support platform and the fixed
reference.
10. The beam-steering device of claim 1, further comprising: a
first passive linkage extending along the first rotational axis and
coupling the support platform, on a side of the support platform
opposite the first linkage, in a pivotable manner to a fixed
support; and a second passive linkage extending along the second
rotational axis and coupling the support platform, on a side of the
support platform opposite the second linkage, in a pivotable manner
to the fixed support.
11. The beam-steering device of claim 1, further comprising: a
third galvanometer situated on the first fixed rotational axis but
on a side of the support platform opposite the first linkage; a
third linkage extending along the first rotational axis but on a
side of the support platform opposite the first linkage, the third
linkage coupling the third galvanometer to the support platform; a
fourth galvanometer situated on the second fixed rotational axis
but on a side of the support platform opposite the second linkage;
and a fourth linkage extending along the second rotational axis but
on a side of the support platform opposite the second linkage, the
fourth linkage coupling the fourth galvanometer to the support
platform.
12. The beam-steering device of claim 11, wherein: the first
galvanometer is wired for rotation, when electrically energized, in
a first rotational direction; the third galvanometer is wired for
rotation, when electrically energized, in a third rotational
direction that is opposite the first rotational direction; the
second galvanometer is wired for rotation, when electrically
energized, in a second rotational direction; and the fourth
galvanometer is wired for rotation, when electrically energized, in
a fourth rotational direction that is opposite the second
rotational direction.
13. The beam-steering device of claim 1, further comprising a
heat-sink in thermal contact with the first and second
galvanometers to conduct heat from the first and second
galvanometer.
14. The beam-steering device of claim 1, wherein: the first linkage
comprises a first section and a first compressible member located
between the first linkage and the support platform, the first
compressible member being configured to provide a variable length
of the first section and the first compressible member; and the
second linkage comprises a second section and a second compressible
member located between the second linkage and the support platform,
the second compressible member being configured to provide a
variable length of the second section and the second compressible
member.
15. The beam-steering device of claim 14, wherein: the first
linkage is substantially non-compliant in rotation about the second
rotational axis and has a variable length to permit rotation of the
support platform about the first rotational axis; and the second
linkage is substantially non-compliant in rotation about the first
rotational axis and has a variable length to permit rotation of the
support platform about the second rotational axis.
16. The beam-steering device of claim 1, wherein: the support
platform comprises first and second inserts coupled to the support
platform, the first insert extending along the first rotational
axis, and the second insert extending along the second rotational
axis; the first linkage comprises a first connecting interface
configured for receiving the first insert; and the second linkage
comprises a second connecting interface configured for receiving
the second insert.
17. The beam-steering device of claim 16, further comprising: a
first pin configured to secure the first insert to the first
connecting interface, the first pin extending, substantially
perpendicularly to the first rotational axis, through a slot in the
first insert and affixed to the first connecting interface; and a
second pin configured to secure the second insert to the second
connecting interface, the second pin extending, substantially
perpendicularly to the second rotational axis, through a slot in
the second insert and affixed to the second connecting interface;
wherein each slot is curved in a manner allowing the respective pin
to engage the respective insert at any of various positions in an
angular range within the curved slot.
18. The beam-steering device of claim 1, wherein the reflective
surface is of a reflective element mounted to the support
platform.
19. The beam-steering device of claim 1, wherein the support
platform is rotatable by the galvanometers over a total mechanical
angle of at least four degrees about each of the rotational
axes.
20. The beam-steering device of claim 1, wherein the support
platform is rotatable by the galvanometers at a frequency of at
least 1.5 kHz.
21. The beam-steering device of claim 1, further comprising: a
first flexure mounted to the first linkage between the support
platform and the first galvanometer; and a second flexure mounted
to the second linkage between the support platform and the second
galvanometer.
Description
FIELD
This disclosure relates to, inter alia, light-beam steering, in
which light from an illumination source is directed at high rates
to a destination. Particular applications include laser-scanning
confocal microscopy, optical scanning, optical particle tracking,
and light-projection systems.
BACKGROUND
A number of emerging technologies are incorporating photonics.
Among these are optical imaging, telecommunications, entertainment
devices, image-projection systems, medical diagnosis and treatment,
photolithography, materials inspection, biosensors, and
surveillance. Applications of these technologies share a
requirement for the rapid and accurate scanning of a laser beam
either to image an object or to project light onto a surface.
When a dynamic system or process is imaged optically, the rate of
image acquisition (i.e., number of image frames acquired per unit
time) is an important consideration. A "dynamic system" may be, for
example, a stationary object that changes over time, a specimen
that moves spatially within a field-of-view, or both processes
occurring simultaneously. Many important processes occur within
time domains that are less than one second. In such cases, it is
frequently desirable to acquire images in at least two spatial
dimensions as rapidly as is consistent with the sampling of
sufficient numbers of photons to form an acceptable image.
Many imaging applications of dynamic systems and processes also
require optimal spatial resolution. Laser-scanning confocal
microscopy is commonly used to improve this parameter, particularly
in the z-dimension that typically extends parallel to the optical
axis. In a scanning-microscope system, such as a laser-scanning
confocal microscope system, the illuminating light or specimen must
be moved relative to the other, or moved relative to one another.
This can be accomplished by moving the specimen while keeping the
illuminating light in a fixed position, by moving the illuminating
light across the specimen while the latter is kept stationary, or
by simultaneously moving both the illumination light and the
specimen.
Certain optical advantages can be achieved by keeping the
illumination light stationary and moving the specimen (for example,
see U.S. Pat. No. 3,013,457, incorporated herein by reference,
which provides an original description of a confocal optical
system). However, this approach involves accelerating, moving, and
decelerating the relatively large mass of a microscope stage or
other type of inspection platform, which typically prevents
scanning at rates greater than a few frames per second. In
addition, this approach restricts or prevents the use of immersion
objectives, in which an intermediate layer of an appropriate
medium, such as oil, water, or glycerin, is maintained between the
objective and the specimen.
Because of such limitations, it is common to scan the beam of
illumination light (typically a laser beam) over the specimen in a
two-dimensional raster manner (involving one-dimensional lines
repeated with intervening steps in the orthogonal dimension) in the
majority of modern scanning microscopes. The laser beam is scanned
by a beam-steering device comprising multiple mirrors mounted on
respective devices capable of controlled motion, such as
galvanometers or piezoelectric elements, or using micro-mirrors
mounted on microelectromechanical systems (MEMS). Another
beam-steering approach utilizes stationary devices, such as
acousto-optical beam deflectors (AODs) that exploit changes in
refractive index of a material to alter the path of the light beam.
However, each of these beam-steering devices is constrained by
limitations related to their maximum achievable scan rates and/or
optical properties.
Galvanometers are currently the beam-steering device most commonly
employed in scanning optical systems. Respective mirrors, mounted
on two independent galvanometers, are used to achieve beam steering
in two (x and y) spatial dimensions. Closed-loop galvanometer pairs
have been used most frequently; these devices exploit the ability
to modulate and control the position of each mirror as it is moved
back and forth in a single dimension in an accurate manner that is
inherent to this type of device. A closed-loop galvanometer
typically also has position-feedback signals that can be used to
verify the position of the mirror at a given point in time.
However, the frequency response of this type of galvanometer is
limited (generally to less than 1 kHz) by several factors, and this
limitation restricts the galvanometer's image-acquisition rate to
typically less than video rates. These factors include the extent
of mechanical movement of the mirror and the size (and hence the
mass) of the reflective surface required. Ultimately, the time
required to dissipate heat resulting from the electromagnetic
forces used to drive movements of the mirror becomes limiting. All
of these factors are inversely related to the frequency response of
the galvanometer system.
In another approach, resonant galvanometers, which have
lower-friction movements, can be driven at frequencies of up to 8
kHz. Such galvanometers have been used to deflect a laser beam in
one spatial dimension. A slower (30-60 Hz) closed-loop galvanometer
is used to deflect the beam in the second spatial dimension. Using
this combination of galvanometers, acquisition rates of 30-60
frames/sec have been achieved for two-dimensional images. For
examples, see Tsien and Bacskai, "Video-Rate Confocal Microscopy,"
in Pawley (ed.), Handbook of Biological Confocal Microscopy, 2nd
ed., chapter 29, Plenum Press, New York, 1995, and U.S. Pat. No.
5,283,433, incorporated herein by reference.
Since prior-art beam-steering systems utilize two mirrors to
achieve both x- and y-direction scanning, these systems cannot
place the axis of a primary deflection surface in a telecentric
conjugate image plane. The need to utilize physically separate
mirrors in galvanometer-based systems to steer the laser beam in
two spatial dimensions in galvanometer-based systems imposes
optical limitations (e.g., see the discussion by Stelzer, "The
Intermediate Optical System of Laser-Scanning Confocal
Microscopes," in Pawley (ed.), Handbook of Biological Confocal
Microscopy, 2nd ed., chapter 9, Plenum Press, New York, 1995). In
imaging situations, in which laser-scanning confocal microscope
systems utilizing single-photon excitation are used, it is
necessary to sense light originating in the sample, such as
fluorescent or reflected light, using a fixed-spot detector such as
a photomultiplier tube or photodiode. To focus light from the
sample onto a fixed point, the light must be de-scanned by the
beam-steering device. Such de-scanning is optimal whenever the axis
of the primary deflecting surface is placed at a telecentric
conjugate image plane. However, such placement is not possible if,
as in the prior art, separate reflective surfaces are used to
deflect the beam in each of the two respective dimensions.
Placement of the reflective surfaces in an axial parallel
arrangement reduces, but does not eliminate, the associated optical
distortion.
Another conventional approach to rapid, single-axis laser-beam
deflection involves the use of an acousto-optical beam deflector
(AOD). As noted previously, this device exploits induced changes in
refractive index of a material to deflect the beam rapidly (with a
1-5 kHz frequency range) in one spatial dimension (e.g.,
x-dimension). As is the case for the resonant galvanometer, a
second device is required to deflect the beam in the second spatial
dimension (e.g., y-dimension). In addition, although scanning
systems utilizing AOD devices have achieved high scan rates over a
somewhat limited range of deflection angles, use of the AOD
introduces optical disadvantages, particularly when used with
laser-scanning confocal microscope systems. These disadvantages
include reduced transmission efficiency, wavelength-dependent
angles of deflection, and the inability of light emitted from the
sample at wavelengths greater than that shone on the sample (e.g.,
fluorescence) to be de-scanned by the AOD device along the optical
path used by the illuminating light. Additional optics are required
to reduce the impact of these disadvantages on spatial resolution,
which decreases the optical efficiency that can be achieved.
High-rate (1-10 kHz frequencies), 2-axis beam deflection has been
achieved using an electrostatically actuated MEMS micro-mirror
beam-steering device. However, the low level of torque produced by
these devices limits the size of the reflective surface to
typically <1 mm. Such a small clear aperture limits the
achievable spatial resolution to much less than that of confocal
systems currently available commercially and places important
limitations on the properties of the intermediate optical system
that can be used. Increasing the size of the mirror results in a
marked reduction in scan frequency and an increase in the dynamic
deformations of reflective surfaces. These deformations diminish
the quality of the reflected light and, thus, the optical quality
of acquired images.
Thus, there is currently a need for a two-axis beam-steering device
having a single, large reflective surface.
SUMMARY OF THE INVENTION
Among various aspects disclosed herein is one aspect directed to
two-axis beam-steering devices having large clear apertures. The
beam-steering devices can deflect laser beams or other illumination
beams in two dimensions with a frequency response in the kHz range.
Micro-machined and/or semiconductor structures can be used to form
a reflector platform hybridized with closed-loop galvanometers to
achieve rapid beam-steering movements. The beam-steering devices
advantageously permit the use of galvanometer actuators that have
suitable torque-generating capabilities to drive a single
reflective surface in two spatial axes (e.g., x- and y-axes).
Galvanometers are particularly well suited for being driven by
amplitude-modulated sine waves employed as mirror-position command
signals, as described in U.S. Pat. No. 7,009,172 to Applicants,
issued Mar. 7, 2006, and incorporated herein by reference in its
entirety, to extend the achievable frequency ranges. In addition,
the use of a single reflective surface operating in a dual-axis
mode optimizes the achievable spatial resolution. The devices can
be configured to generate dual-axis position-feedback signals that
are usable for monitoring and further increasing the accuracy of
beam-steering.
According to another aspect, these beam-steering devices are
incorporated into a laser-scanning confocal microscope system
(LSCMS), as an exemplary system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of an embodiment of a beam-steering device
comprising two galvanometer actuators coupled to a single
reflective element (e.g., mirror) and including an
optical-position-feedback system in x- and y-axes located under the
reflective element.
FIG. 2 is a plan view of an embodiment of a beam-steering device
comprising two galvanometer actuators coupled to a single mirror
and including capacitive-feedback devices for controlling motion of
the mirror in x- and y-axes.
FIG. 3(A) is a plan view of an embodiment of a beam-steering device
comprising four galvanometer actuators coupled to a single
mirror.
FIG. 3(B) is a plan view of an embodiment of a beam-steering device
comprising two galvanometer actuators with opposing linkages on
opposite sides of the mirror.
FIG. 4(A) is a perspective finite-element-analysis diagram of an
exemplary configuration of the mirror-platform region in which
linkages coupled from actuators to the mirror comprise widely
separated flexures.
FIGS. 4(B)-4(D) depict alternative shapes for the flexures in the
linkages.
FIG. 5 is a perspective view of an embodiment of the
mirror-platform region comprising in which four slip-joint linkages
with internal pins.
FIG. 6(A) is a schematic plan view of the slip-joint beam-steering
structure shown in FIG. 5.
FIG. 6(B) schematically depicts an exemplary method by which an
appropriate slot can be defined within a platform-linkage
insert.
DETAILED DESCRIPTION
A representative embodiment of a two-axis, beam-steering device
desirably has the following characteristics and properties: (a)
comprises a reflective element (e.g., a mirror) having a highly
reflective surface (produced, for example, using metal and/or
dielectric coatings); (b) exhibits minimal spatial deformation
either under static conditions or as a result of dynamic movements;
and (c) driven by at least two galvanometer actuators each having a
rotational axis; and (d) the rotational axes are substantially
orthogonal to each other. Desirable (but not required) performance
characteristics include: (1) deflects a beam of incident
electromagnetic radiation over a total angle of at least 4.degree.
(mechanical); (2) deflects the beam at a frequency of >1.5 kHz;
(3) the reflective element has a width of at least approximately 3
mm and a high reflective-surface fill factor (desirably close or
equal to 100%); and (4) the galvanometers are drivable by any of
various types of command signals, including (but not limited to)
signals used for raster scanning and signals comprising
amplitude-modulated sine or modified sine-wave functions. The
representative embodiments described below meet or exceed these
criteria.
According to various embodiments, the torque required to move a
reflective element having significant mass, especially a reflective
element mounted to a support platform or the like, through a
significant mechanical angle (e.g., at least 4.degree.) is achieved
using at least two closed-loop galvanometers. An example of a
suitable galvanometer is Model 6215 commercially available from
Cambridge Technology (Cambridge, Mass.). The galvanometers are
driven by appropriate driving and control circuitry, according to
the manufacturer's specifications and requirements. Since
galvanometers tend to generate heat during operation, it is
desirable to cool them at least passively and more desirably
actively. With appropriate modifications to the electronic control
circuitry used to drive the galvanometers, active cooling of the
galvanometers to increase the rate of thermal transfer from the
galvanometers during use, and use of "intelligent control"
non-raster command signals, the frequency response of the
galvanometers can be extended to 5 kHz and higher.
A first representative embodiment of a beam-steering device is
shown in FIG. 1, comprising two galvanometers 101, 102 coupled to a
support platform 104a. The galvanometers 101, 102 are situated
orthogonally to one another. Each galvanometer 101, 102 comprises a
respective central shaft 105. As the shafts 105 are rotated, energy
is transmitted to the support platform 104. Operation of the
galvanometers 101, 102 generates heat that can be rapidly conducted
away from them in an active manner using a liquid-cooled heat sink
103. The heat sink 103 comprises a block 103a that defines a
chamber 103b and includes an inlet 108 for conducting cooling
liquid into the chamber and an outlet 10, and an outlet 109 for
conducting coolant liquid from the chamber to an external
temperature-controlled coolant circulator (not shown). The housing
of each galvanometer 101, 102 is situated in or at least is
thermally coupled to the chamber 103b so that, as the coolant
liquid (e.g., water or other suitable liquid) flows through the
chamber 103b, the liquid draws heat from the galvanometers 101,
102. The external temperature-controlled coolant circulator cools
the liquid to the desired temperature and returns the liquid to the
chamber 103b desirably continuously during operation of the
galvanometers. For efficient operation, the block 103a can be made
of aluminum alloy or other appropriate material having a suitable
thermal-transfer coefficient. The block 103a can serve both to
dissipate heat from the structure and to provide structural support
for the galvanometers and other components of the device.
A reflective element 104b (e.g., a mirror) is attached to the
support platform 104a. The reflective element 104b has an obverse
reflective surface 104c and a reverse surface (facing the support
platform 104a), and can have any of various profiles (e.g., round
or square). The reflective element 104b is mounted (via its reverse
surface) to the support platform 104a. The reflective element 104b
need not be the same size or the same shape as the support platform
104a. The center of the reflective surface 104c desirably is
situated substantially at the center of the support platform
104a.
The first galvanometer 101 rotates the support platform 104a (and
hence the attached reflective element 104b) about the x-axis as a
light beam is incident on the reflective surface 104c (desirably at
the center of the reflective surface). This rotation of the support
platform 104a causes the reflective element 104b to "steer" (by
reflection) the incident beam in the x-direction. The second
galvanometer 102 rotates the support platform 104a (and hence the
attached reflective element 104b) about the y-axis as a light beam
is incident on the reflective surface 104c (desirably at the center
of the reflective surface). This rotation of the support platform
104a causes the reflective element 104b to steer (by reflection)
the beam in the y-direction. The combined movements provided by
both galvanometers 101, 102 produce a combined tip-tilt motion of
the support platform 104a (and reflective element) that causes the
reflective element 104b to deflect the beam simultaneously in both
the x- and y-axes.
The respective shaft 105 of each galvanometer 101, 102 extends
along the respective tilt axis and is attached via a respective
linkage 106 to the support platform 104. Each linkage 106 has high
stiffness (and thus is substantially non-compliant) with respect to
the torque applied to the linkage by the respective shaft 105 in
the direction of rotation of the shaft about the respective axis,
but has significant compliance in a direction that is orthogonal to
the plane of the page. Thus, the linkages 106 permit movements of
the distal portions of the shafts 105 in the orthogonal axis (i.e.,
in and out of the plane of the page of FIG. 1). These movements
require that the shafts 105 be capable of making small changes in
axial length, which are achieved by including one or more
respective spring mechanisms 107 (called "flexures" herein) on each
shaft between the linkage 106 and the distal end of the shaft. The
distal end of each shaft 105 is coupled to the support platform
104a so as to cause movement of the support platform corresponding
to respective rotational motions of the shafts.
The support platform 104a can be made of any suitable rigid
material, including any of various metals, silicon, ceramic, glass,
polymer, and the like. The support platform 104a need not be
separate from the reflective element 104b, wherein the reflective
element is mounted to the support platform. Alternatively, the
reflective surface 104c can be formed directly on the surface of
the support platform 104. In addition, the support platform 104a
can be contiguous with the linkages 106.
As noted above, a beam of light is incident on the reflective
element 104b from a direction such as from above the plane of the
page of FIG. 1. As the galvanometers 101, 102 impart rotational
motions to the shafts 105, corresponding motions are imparted to
the reflective element 104b, which causes deflection of the beam.
The actual position of the deflected beam in two-dimensional space
can be determined from reflector-position feedback signals. During
imaging applications, these signals can be recorded concomitantly
with intensity values obtained for light originating from the
sample that is being illuminated by the deflected beam.
The reflector-position feedback signals can be generated in several
ways. For example, the signals can be produced by monitoring
galvanometer-shaft positions. Many types of galvanometers are
equipped with shaft-position feedback sensors that provide such
signals. A more accurate feedback signal (i.e., a signal that more
closely corresponds to the actual direction of beam deflection from
the reflective element 104b) can be produced by optically detecting
the position of light that has been reflected from the reflective
surface 104c and correlating the light-position data with position
of the reflective element 104b. Alternatively, accurate feedback
signals can be obtained by making capacitive measurements between a
fixed platform 110 (or analogous structure; shown in phantom
outline and removed to reveal underlying structure) and the
reflective element 104b mounted on the support platform 104a. The
latter configuration is illustrated schematically in FIG. 1, in
which the embodiment includes sensing elements 115 for sensing the
x-position of the reflective element 104b and sensing elements 116
for sensing the y-position of the reflective element. In this
embodiment the sensing elements 115, 116 are located beneath (as
viewed in the figure) the support platform 104a.
Yet another alternative position-sensing technique that provides
feed-back data involves measurement of capacitive changes between
electrostatically-charged silicon comb-finger structures (not
shown) attached to the support platform 104a and accompanying
motions of the platform. This position-sensing technique has been
used by Milanovic et al., "High-Aspect-Ratio Two-Axis Scanners in
SOI," 16th IEEE International Microelectromechanical Systems
Conference, pp. 255-258, (2003), and is illustrated in FIG. 2, in
which the support platform 104a and fixed platform 110 are made of
silicon or other semiconductor material on which respective pairs
of interdigitated silicon comb-fingers are formed for sensing
motions in the x-direction (comb-fingers 113) and for sensing
motions in the y-direction (comb-fingers 114) of the support
platform 104a. As the support platform 104a undergoes tilts in the
x- and y-directions, corresponding capacitance changes occur
between the comb-fingers 113, 114, respectively. The magnitude of
the capacitance change is proportional to the corresponding
tip-angle of the support platform 104a.
In each of the position-sensing feedback schemes described above,
electrical signals pertaining to the position of the support
platform 104a are conducted or otherwise transmitted to external
circuitry (e.g., a controller) via bond-pads 111 (or analogous
structures) for x-direction sensing and bond-pads 112 (or analogous
structures) for y-direction sensing. A controller (not shown) is
advantageous because it can perform data analysis and processing of
data contained in the signals produced by the sensors. This
processing desirably yields real-time return-control signals to the
galvanometers so that the angles of tilt of the reflective element
about the two rotational axes and the frequency of tilt are as
desired.
For motions of the support platform 104a in the x- and
y-directions, it may be necessary or desirable to generate more
torque than can be supplied by a single galvanometer in each
respective direction, or it may be necessary or desirable to
increase the frequency response of the beam-steering devices
further. To such end, at least one respective additional
galvanometer can be added for augmenting motions in each direction.
An example of this configuration is shown in FIG. 3, in which the
device comprises two opposing galvanometers along the same
rotational axis for each direction of motion. Specifically, the
galvanometers 201, 205 provide tilt motions about the x-axis, and
the galvanometers 202, 204 provide tilt motions about the y-axis.
Note that the galvanometers of each pair are located on opposing
sides of the reflective element 104b, and that the respective axes
A.sub.1, A.sub.2 of the galvanometers 201, 205 and 202, 204
intersect each other beneath the center of the reflective element
104b. Use of two respective galvanometers for motion in each of the
x- and y-directions effectively doubles the torque applied to the
support platform 104a. Also, placing the galvanometers of each pair
in an opposing manner as shown stabilizes the position of the
support platform 104a and reflective element 104b by generating
symmetrical loads to the support platform. To achieve coordinated
motion of the galvanometers of each pair, the opposing
galvanometers 201, 205 and 202, 204 are wired oppositely (i.e., the
+ and - leads are reversed). Consequently, the galvanometers of
each pair receive the same applied voltage but the shafts of each
pair rotate in opposite directions (clockwise versus
counter-clockwise) on the respective axis (i.e., galvanometer 202
is wired oppositely of galvanometer 204 for motions about the
x-axis, and galvanometer 201 is wired oppositely of galvanometer
205 for motions about the y-axis).
Use of pairs of galvanometers can require reconfiguring the cooling
block 203 to extend to each of the galvanometers 201, 205 and 202,
204, as shown.
Use of multiple respective galvanometers for motions in each of the
x- and y-directions can be applied to any of various embodiments of
beam-steering devices. However, use of only one respective
galvanometer for each motion is a lower-cost option if a maximal
frequency response or maximal torque is not necessary for a
particular application. In certain configurations comprising a
total of only two galvanometers (one for x-direction motion and one
for y-direction motion), it is advantageous to stabilize, along the
respective tilt axis, at least one of the sides of the support
platform or reflective element opposite the respective
galvanometer. An embodiment of this configuration is shown in FIG.
3(B), which depicts two galvanometers 201, 202, the support
platform 104a, the mirror 104b, and linkages 107a between the
galvanometers and the support platform 104a. On opposite sides of
the platform 104a from the galvanometers are second linkages 126,
flexures 107b, and shafts 120. The shafts 120 are journaled in
respective bearings 124 in respective side walls 122 (or analogous
structures). Each second linkage 126 desirably is identical to the
respective opposing linkage 106 on the galvanometer side and is
coupled along the respective axis A.sub.1, A.sub.2 to the support
platform 104a or reflective element 104b. Each second linkage 126
is passive and rotates freely (with or without bearings 124) as
required to stabilize the non-galvanometer side(s) of the support
platform 104a.
FIG. 4(A) depicts a square reflective element 136 mounted on a
correspondingly shaped support platform 226. The depiction is in a
form suitable for finite-element analysis (FEA) of the mechanism.
The support platform 226 is supported by linkages 225 on each
corner of the platform 226. The linkages 225 are widely separated
from each other with respect to each tilt axis A.sub.1, A.sub.2 and
represent respective linkages to respective galvanometers (not
shown). In this embodiment, the linkages 225 can be contiguous with
the support platform 226 (e.g., the support platform 226 and
linkages 225 can be made from a single wafer of silicon or a
suitable metal). The ends of the respective galvanometer shafts
coupled to the linkages 225 are represented by rectangular blocks
227. Splitting the linkages 225 in each tilt direction into two
components as shown and spacing them as far apart as possible
(within the dimensions of the support platform 226), improves the
transfer of rotational energy from the galvanometer shafts 227 to
the support platform 226 without significantly sacrificing linkage
compliance to allow movements in the orthogonal axis.
FIGS. 4(B)-4(D) are respective plan views showing, with respect to
the configuration shown in FIG. 4(A), alternative configurations of
linkages. FIG. 4(B) depicts the support platform 226 coupled by a
total of eight flexures 225b to four galvanometer shafts 227. By
utilizing two galvanometer attachments in each of the x- and
y-dimensions, wherein the attachments are displaced from the
respective centers of rotation, a more efficient translation of
rotational torque is achieved, while allowing for sufficient flex
in each direction. Also, increased mechanical stability is realized
(i.e., reduced flex) of the support platform 226 with the increased
number of attachments. FIG. 4(C) shows a configuration similar to
FIG. 4(B), except that both the thickness and the length-to-width
ratio of the flexures 225c are different than in FIG. 4(B). These
variables can be varied to affect stiffness in any of various
desired directions. FIG. 4(D) shows that the flexures 225d can have
any of various geometric configurations. The substantially
elliptical shape of the flexures 225d reduces concentrations of
stress that otherwise would be present near sharp angles.
Another representative embodiment 400 of a support platform 402,
for supporting a reflective element, and linkages 404 coupled to
the platform is illustrated in FIG. 5. In the embodiments described
above, most of the tilt energy imparted by the galvanometers to the
platform is converted to potential energy stored by deformations of
the linkages. But, care must be taken so as not to cause permanent
deformation (e.g., breakage) of the linkages. The amount of
potential energy absorbed by the system can be described in terms
of the degree of deformation and the spring constants of the
flexures of the linkages that couple the galvanometers to the
platform. Generation of potential energy (i.e., energy generated by
the galvanometers) can be substantially reduced by allowing linkage
surfaces to move relative to one another. Assuming low frictional
losses, such a configuration can, at least under some conditions,
produce more rapid movements of the platform for a given input
power. A possible disadvantage of this approach is the possibility
of imparting detrimental wear on the moving surfaces during
prolonged use.
In the embodiment 400 of FIG. 5, tilt commands are transferred in
the x- and y-axes while simultaneously allowing tilt motions in
both axes. In this embodiment the rotational shaft of each of four
galvanometers (not shown but arranged as shown in FIG. 3(A)) has a
respective "U-shaped" end 220 configured to accept an insert 221
coupled to the support platform 402. Each insert 221 includes a
respective pin 206 that fits through both a hole in the respective
end 220 and through a slot 222 in the insert 221. The resulting
coupling stabilizes the support platform 402 by limiting unwanted
platform movements relative to the shaft of the respective
galvanometer. Each U-shaped end 220 transfers rotational energy
from the galvanometer shaft to the support platform 402.
Collectively, the ends 220 and inserts 221 form respective
rotational translators that allow simultaneous rotation of the
support platform 402 in the x- and y-axes. The break-out diagrams
at the bottom of FIG. 5 show a perspective view of an insert 221, a
U-shaped end 220, and a pin 206.
FIG. 6(A) is a schematic plan view of the beam-steering device
illustrated in FIG. 5. In FIG. 6(A), the galvanometers 215-218 are
depicted as respective rectangular blocks. The galvanometer 215
(and optionally 218) generates torque for moving the support
platform 402 and reflective element 403 about the x-axis, and the
galvanometer 216 (and optionally 217) generate torque for moving
the support platform 402 and reflective element 403 about the
y-axis. The slots 222 (FIG. 5) defined in the inserts 221 desirably
have a curvature that is suitable for accommodating corresponding
rotational motions of the support platform 402. In FIGS. 6(A)-6(B)
the vertical dashed lines that project from the left-most insert to
the side-view of the insert shown in FIG. 6(B) indicate an
exemplary manner in which the radially curved slot can be formed.
The radius of curvature corresponds to the distance from the slot
to the center of the support platform 402. The bold circle 211
represents a hypothetical slot that would allow the support
platform 402 to rotate completely 360.degree. (mechanical). In
practice, the angle 212 that must be accommodated is typically up
to +15.degree. (mechanical). The portion of the bold circle 211
within this angular range corresponds to the material that must be
removed from the insert to form the slot depicted in the insert
221.
Finite-element analysis can be used to optimize dimensions and
select appropriate materials for fabricating the linkages for the
size and mass of the support platform and reflective element that
is needed for a particular application. FIGS. 4 and 5 were
generated, and the motions of the depicted devices were simulated,
using ANSYS (Canonsburg, Pa.), an exemplary finite-element-modeling
software tool. The linkages, support platform, and other components
of the beam-steering device can be constructed from silicon using
well-established, silicon-on-insulator (SOI) fabrication
techniques. Alternatively, components can be created using
micromachining techniques such as those similar to methods employed
in the fabrication of parts for watches. Mirrored surfaces of
reflective elements can be fabricated from a single crystalline
silicon substrate or from a polished metal plate. Alternatively, a
commercially available conventional thin-metal or dielectric-coated
reflector can be attached to the support platform.
Whereas the invention has been described in connection with
multiple representative embodiments, it will be understood that the
invention is not limited to those embodiments. On the contrary, the
invention is intended to encompass all modifications, alternatives,
and equivalents as may be included within the spirit and scope of
the invention, as defined by the appended claims.
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